Smoldering powders, so-called nests, i.e. particulate burning embers and agglomerates, are common ignition sources in dust explosions and fires, and pose a serious threat in powder drying systems, such as spray dryers, that produce powders within food, pharmaceuticals, dairy, chemical, agrochemical, energy, biotechnology, healthcare and many more, producing e.g. milk powder, coffee whitener, infant formula, coffee powder, pharmaceutical, chemical, etc. During a spray drying process deposits may occur when generally not-high-temperature-droplets accumulate in smaller or larger deposits on walls or floors of driers. The product temperature of such deposits may rise due to intrinsic chemical reactions, which may cause the powder to smolder, or even burn when in oxygen filled atmospheres. During smoldering, CO gas production starts, and an oxidation and/or pyrolysis reaction occurs. Lumps of deposited smoldering product may typically develop inside the spray-chamber and/or air disperser/and/or atomizer, cyclone or in an interior or exterior fluid bed, or in exterior bag filters.
If powder deposits are not removed from the drying system and reach a certain size, they may form lumps and fall down inside the system, even travel through the process and break-up so a so-called glowing mass, initiated by heat as an exothermal oxidation reaction. The glowing mass may become exposed and ignite a powder filled atmosphere at some point in time. Another scenario is that larger lumps of powder are subjected to heat, e.g. at the bottom of a fluid-bed and smoldering reactions are activated. This can take place also in low oxygen atmospheres or the like. Subsequently, the smoldering lump may travel through the process and finally result in plant fires or dust explosions. Primary fire prevention is typically based on temperature surveillance and regular cleaning practices. Often, such primary prevention is not fast enough due to process reaction delay and therefore insufficient; and additional measures are implemented in the form of smoldering detection systems for early warning and protection of the drying system.
One prior art solution is produced and marketed by FIKE under the name WarnEx. This system consists of multiple so-called sampling and detection units (SDUs) placed on each inlet and outlet of a powder drying system and a control unit processing the signals received from the SDUs. The WarnEx system thus employs extraction of samples in the sampling part of an SDU and transport of the detection result to the detection part of the SDU through suitable electric cabling.
DE 202014101777 U1 describes a spray dryer with a humidity detector having a measuring device, which humidity detector may also be used for detection of CO gas. The specific type of measuring device and measuring method employed in detection of CO gas by means of the humidity detector is, however, not mentioned.
Furthermore, DE 202014101777 U1 and other known CO gas detection systems for spray dryers employ extraction of gas samples from the flow at least at an inlet and at an outlet thereof, and transport these gas samples via e.g. Teflon® tubes to a common IR laser or a common NDIR detector system (not laser) provided at a distance from the chamber. Such CO gas detection systems are produced and marketed by e.g. Hobré Instruments and ATEX CO.
However, the known CO gas detection systems have several disadvantages, including:                reduced result reliability when measuring on a small volume of gas in a gas sample,        the sample may not be representative for the actual CO gas concentration in the flow out or in as it depends on the airflow and dilution at the selected extraction position,        they require accurate timing between the physical gas sample from the in/outlet in the system,        need for frequent calibration to ensure the required sensitivity at all times, in particular when the flow rates or volume size of drier changes,        high acquisition, installation, operation, maintenance and verification/calibration costs, and        is difficult to adapt to changes in flow or volume size of dryer, and        requires external control signals from another source.        
Furthermore, the prior art systems, such as those of ATEX CO and Hobré, as mentioned employ tubing to transport the gas samples from the sampling site to the measurement site, i.e. the common external detector. Such tubing requires maintenance, may leak, is ergonomically difficult to handle when installing and requires a large detector system cabinet and high power consumption. Additionally, their system cabinet (of the floor standing type) requires certain conditions for the installation area, like the temperature shall be below 25° C., it has to be installed in a clean confined environment and access to the cabinet from several sides, and a water drain is necessary for drainage of condensate coming from the system. Also, humidity in the samples must be removed before measurement, and it is needed for the detector system calculation to be compensated for e.g. different geometries or flows inside the tubes, so that repairs on tubes and connectors cannot be performed by plant staff but must be undertaken by professionals, e.g. detector providers.
The response time of a CO gas detection system should be low and preferably real time and the sensitivity high in order to provide an effective early warning system.
However, the transport time of the air samples, added to the detector purge time, analysis time, calculation time and the intrinsic holding time in the common measurement chamber adds up to longer response times for these prior art systems of around 15-60 seconds. It is desirable to decrease this response time, which may prove too long to constitute early warning. As for sensitivity, some of the prior art systems are incapable of detecting CO gas concentrations of generally below about 1 ppm and for the best prior art systems below about 0.4 ppm. To provide for an effective early warning system it is, however, desired to provide a sensitivity enabling detection of CO gas concentrations generally being below 0.4 ppm and preferably below 0.1 ppm.
During initial commissioning of prior art CO gas detection systems for spray dryer systems, tests are needed where hazardous CO gas are injected into the entire dryer system for testing the gas retention time for necessary calibration procedure of the CO gas detection. Importantly, the purchase and handling of CO gas normally requires local regulatory approval. With some of the prior art systems as stipulated by the spray dryer systems risk assessment or to ensure optimal system performance require an enhanced test carried out frequently on the system before a spray dryer system is released for operation. One of the steps in this test is use of a special certified test gas consisting of N2+CO (CO=8.0 ppm) to confirm that the CO gas detection is measuring correctly. The test gas is expensive and not commonly available on the market. Another requirement is leak tests of sample tubes for prior art CO gas detection systems. This test must frequently be carried out to ensure that the CO gas detection system actually is measuring the process gas coming from inside the dryer system.
A further problem resides in the presence of invading spurious CO gas from external sources, in particular coming from climate fluctuations, variations in ambient air or exhaust gases from cars, field burning of crops, human activity, etc., which causes the ambient CO gas, which enters into the system air, to vary greatly according to location and pollution in the plant, area, country and weather. Many decades larger CO gas content may thus be present in the inlet air and mask or overpower the CO gas generated from any smoldering powder inside the process. This problem is attempted solved for the prior art systems by using a reference ambient air sample and/or by calculating the difference between CO gas content in inlet(s)— and outlet(s) samples using a fixed reference CO gas value or a differential calculation, i.e. a subtraction of the summed CO gas contents from all inlets and the summed CO gas contents from all outlets.